[PATCH RFC 05/11] rust: sync: Add dummy LockClassKey implementation for !CONFIG_LOCKDEP

Asahi Lina
2023-07-14T09:31:42Z

Lock classes aren't used without lockdep. The C side declares the key
as an empty struct in that case, but let's make it an explicit ZST in
Rust, implemented in a separate module. This allows us to more easily
guarantee that the lockdep code will be trivially optimized out without
CONFIG_LOCKDEP, including LockClassKey arguments that are passed around.

Depending on whether CONFIG_LOCKDEP is enabled or not, we then import
the real lockdep implementation or the dummy one.

Signed-off-by: Asahi Lina <lina@asahilina.net>
---
rust/kernel/sync.rs | 29 ++++++++---------------------
rust/kernel/sync/lockdep.rs | 27 +++++++++++++++++++++++++++
rust/kernel/sync/no_lockdep.rs | 19 +++++++++++++++++++
3 files changed, 54 insertions(+), 21 deletions(-)

diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
index d219ee518eff..352472c6b77a 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -5,37 +5,24 @@
//! This module contains the kernel APIs related to synchronisation that have been ported or
//! wrapped for usage by Rust code in the kernel.

-use crate::types::Opaque;
-
mod arc;
mod condvar;
pub mod lock;
mod locked_by;

+#[cfg(CONFIG_LOCKDEP)]
+mod lockdep;
+#[cfg(not(CONFIG_LOCKDEP))]
+mod no_lockdep;
+#[cfg(not(CONFIG_LOCKDEP))]
+use no_lockdep as lockdep;
+
pub use arc::{Arc, ArcBorrow, UniqueArc};
pub use condvar::CondVar;
pub use lock::{mutex::Mutex, spinlock::SpinLock};
+pub use lockdep::LockClassKey;
pub use locked_by::LockedBy;

-/// Represents a lockdep class. It's a wrapper around C's `lock_class_key`.
-#[repr(transparent)]
-pub struct LockClassKey(Opaque<bindings::lock_class_key>);
-
-// SAFETY: `bindings::lock_class_key` is designed to be used concurrently from multiple threads and
-// provides its own synchronization.
-unsafe impl Sync for LockClassKey {}
-
-impl LockClassKey {
- /// Creates a new lock class key.
- pub const fn new() -> Self {
- Self(Opaque::uninit())
- }
-
- pub(crate) fn as_ptr(&self) -> *mut bindings::lock_class_key {
- self.0.get()
- }
-}
-
/// Defines a new static lock class and returns a pointer to it.
#[doc(hidden)]
#[macro_export]
diff --git a/rust/kernel/sync/lockdep.rs b/rust/kernel/sync/lockdep.rs
new file mode 100644
index 000000000000..cb68b18dc0ad
--- /dev/null
+++ b/rust/kernel/sync/lockdep.rs
@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Lockdep utilities.
+//!
+//! This module abstracts the parts of the kernel lockdep API relevant to Rust
+//! modules, including lock classes.
+
+use crate::types::Opaque;
+
+/// Represents a lockdep class. It's a wrapper around C's `lock_class_key`.
+#[repr(transparent)]
+pub struct LockClassKey(Opaque<bindings::lock_class_key>);
+
+impl LockClassKey {
+ /// Creates a new lock class key.
+ pub const fn new() -> Self {
+ Self(Opaque::uninit())
+ }
+
+ pub(crate) fn as_ptr(&self) -> *mut bindings::lock_class_key {
+ self.0.get()
+ }
+}
+
+// SAFETY: `bindings::lock_class_key` is designed to be used concurrently from multiple threads and
+// provides its own synchronization.
+unsafe impl Sync for LockClassKey {}
diff --git a/rust/kernel/sync/no_lockdep.rs b/rust/kernel/sync/no_lockdep.rs
new file mode 100644
index 000000000000..69d42e8d9801
--- /dev/null
+++ b/rust/kernel/sync/no_lockdep.rs
@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Dummy lockdep utilities.
+//!
+//! Takes the place of the `lockdep` module when lockdep is disabled.
+
+/// A dummy, zero-sized lock class.
+pub struct LockClassKey();
+
+impl LockClassKey {
+ /// Creates a new dummy lock class key.
+ pub const fn new() -> Self {
+ Self()
+ }
+
+ pub(crate) fn as_ptr(&self) -> *mut bindings::lock_class_key {
+ core::ptr::null_mut()
+ }
+}

--
2.40.1